Over the years, the producers and users of benzene and paraxylene have periodically modified product specifications to require increased purity. Customers now desire extremely high purity products. While these ultra-high purity products can be reliably made on a small scale, it is much more difficult to consistently achieve such purity on a commercial scale. The problem is magnified when using naphtha feedstocks with variable or fluctuating compositions.
Benzene and paraxylene are typically produced from petroleum naphtha by a variety of reforming operations, also referred to herein as "aromatizing" operations. Raw naphtha is typically highly paraffinic in nature, but may contain significant amounts of naphthenes and minor amounts of aromatics or olefins, or both. Often, the naphtha feed is hydrodesulfurized prior to reforming to reduce catalyst poisoning. The objective in reforming is to produce a slate of aromatics, for example, from C6 to C10 aromatics, which can be subsequently further processed to produce the desired aromatics products such as benzene and paraxylene. Reforming includes dehydrogenation, isomerization and hydrocracking reactions. Dehydrogenation reactions typically include dehydroisomerization of alkylcyclopentanes to aromatics, dehydrogenation of paraffins to olefins, dehydrogenation of cyclohexanes to aromatics, and dehydrocyclization of paraffins and olefins to aromatics. Isomerization reactions include isomerization of n-paraffins to isoparaffins, hydroisomerization of olefins to isoparaffins and isomerization of substituted aromatics. Hydrocracking reactions include hydrocracking of paraffins and hydrodesulfurization if any sulfur compounds remain in the feedstock. Hydrocracking reactions are considered undesirable when they result in the formation of low carbon number gaseous products or light ends.
Numerous patents discuss the general concept of splitting a naphtha feed stream into a light fraction and a heavy fraction, then reforming each fraction separately. The following split feed patents are hereby incorporated by reference, to the extent they are not inconsistent with this invention: U.S. Pat. No. 4,897,177; U.S. Pat. Nos. 5,106,484; Re 33,323; 3,957,621; 2,867,576; 2,944,959; 3,172,841; 3,409,540; 4,167,472; 4,358,364; 3,753,891; 4,645,586; 3,280,022; 2,867,576; 3,753,891; 4,401,554; 4,203,826; 3,635,815; 3,499,945; 2,653,175.
Reforming is generally done in the presence of a catalyst. Numerous patents, including some of those listed above, disclose different types of commercially available catalysts that may be used to reform naphtha. Catalysts are also disclosed in Pat. Nos. 4,347,394 and 4,104,320, which are hereby incorporated by reference to the extent they are not inconsistent with the present invention. Often, two different types of catalyst systems are used to produce aromatics in split-feed reforming. The '177 patent discusses the use of non-acidic "monofunctional" catalysts, for reforming light fractions, and acidic "bifunctional" catalysts for reforming heavy fractions.
Bifunctional catalysts have metal sites and strong acid sites. In certain bifunctional catalysts, a metal hydrogenation-dehydrogenation component is dispersed on the surface of a porous inorganic oxide support such as alumina oxide. Additional metallic components, known as promoters, may be added to the platinum metal sites to provide increased activity or selectivity or both. Examples of promoters include iridium, rhenium, tin, and the like. In contrast, monofunctional catalysts are "non-acidic," and have large pore zeolites as supports rather than inorganic oxides such as alumina. Suitable monofunctional catalysts include non-acidic carriers such as a zeolite L, and at least one noble metal of Group VIII. "Nonacidic" or "monofunctional" reforming catalysts are characterized by a substantial absence of accessible acidic sites. The substantial absence of accessible acidic sites can be inferred from the reforming reaction products or determined by various analytical techniques well known in the art. For example, certain bands in O--H stretching region of infrared spectrum of the catalyst can be used to measure the number of acid sites that are present. For purposes of this invention, nonacidic catalysts will include any zeolite based catalyst having a silica/aluminum ratio greater than 500 or having no more than 5.0%, and preferably less than 1.0%, of its exchangeable cation sites occupied by protons. A nonacidic reforming catalyst typically comprises platinum on a substantially non-acidic support. A substantially non-acidic support material has an .alpha. less than about 0.1, where .alpha. refers to the relative n-hexane cracking activity of the support compared to a standard silica/alumina catalyst as determined in the well known Alpha Test, which is described in U.S. Pat. No. 3,354,078 and in the journal "Catalysis," Vol. 4, p. 527 (1965); Vol. 6, p. 278 (1966); and Vol. 61, p. 395 (1980).
In one aspect, the present invention relates to an improved split-feed method of making high purity benzene and high purity paraxylene using a dual catalyst system. While the prior art discloses monofunctional and bifunctional catalysts and split feed processes in general, it fails to disclose or suggest one or more key aspects or advantages of the present invention. For example, the '177 patent identifies several possible light and heavy fractions, including a light fraction of C6-C7. (Col. 4, ln 21.) It also implies that the heavy fraction may include C8+'s. (Col. 4, lns 13-16.) However, the patent states that the preferred light fraction is C6-C8, and thus directs one away from the C7-/C8+ split of the present invention. Moreover, it discloses nothing about the removal of heavy ends from the raw naphtha, nor does it suggest the surprisingly high RON of the present invention, nor does it suggest the cut point adjustment feature of the present invention.
The '323 patent discloses a split feed process. But that process is based on a C6- light fraction and a C7+ heavy fraction. While the light fraction is said to have 10% or more C7+'s, there is no suggestion to provide a C8+ heavy fraction or to restrict C7-'s in the heavy fraction. In fact, the '323 patent states that the C7+ fraction contains "greater than 90%, preferably at least 95%" of C7+'s. (Col. 3, lns 62-64.) Example 1 discloses a heavy fraction containing 91.9% C7 to C9 hydrocarbons. (Col. 7, ln 2.) The catalysts used to reform the heavy fraction are said to be "efficient in converting C7+ hydrocarbons," so that the '323 patent does not at all provide any motivation to provide a C7- light fraction and a C8+ heavy fraction. (Col. 5, lns 40-42.) Likewise, no other features of this invention are disclosed.
A publication by Swift and Moser, entitled "New Options for Aromatics Production," published in 1995, refers to several processes for making benzene and paraxylene. One process involves splitting a full-range naphtha into "light and heavy cuts" and directing the light fraction to the "RZ Platforming" unit and the heavy fraction to the "CCR Platforming" unit (See p. 7 and FIG. 6.) However, there is no disclosure of any particular composition of the heavy fraction that is treated in the CCR unit, much less any suggestion to restrict the amount of C7-'s in the heavy fraction, nor is there any suggestion of other aspects of this invention, including the unusually high RON of the heavy fraction reformate. Furthermore, that articles does not disclose any means for recovering a high purity benzene product, nor a method for accomodating highly fluctuating naphtha feedstock.
The '554 patent discloses a split feed process, but discloses a very broad cut point range of between 200.degree. F. to 350.degree. F. The cut point is said to be preferably the mid-point boiling range of the naphtha. That patent states that the heavy fraction contacts the reforming catalyst for a longer period than the light fraction, so that "for a given severity the degree of reforming reaction for the heavy fraction is much higher than the light fraction." (Col. 5, lns 8-11.) However, in addition to not disclosing or suggesting the C7/C8 split feed aspect of this invention, that patent does not disclose the surprisingly high octane number of this invention. For example, the RON's disclosed are only 96 and 98. (See col. 6, lns 42-43 and Table 3.) Moreover, there is no suggestion to remove light or heavy ends, nor is there any suggestion to adjust the cut point based on benzene or paraxylene limit points.
The '621 patent discusses distillation of a reformate feed stream to provide light and heavy reformate product fractions. However, the light reformate product fraction (containing paraffins and some benzene) is removed as overhead and is not processed any further, e.g., to provide a high purity benzene product (Col. 4, lns 65-68; see FIG. 1, line 12.) Also, the feed stream being subjected to "splitting" is itself a reformate, and already contains a high concentration of aromatics. (Col. 4, lns 57-61.) That patent does not disclose splitting of raw naphtha feed into light and heavy fractions. Example 1 in the patent refers to production at a severity to produce C5+ reformate having 103 RON. Significantly, however, the charge used to produce that reformate is itself the "heavy end of a reformate." (Col. 14, ln 3.) Moreover, the naphtha feed used to produce that reformate feed is C6- naphtha. (Col. 14, ln 4.) Thus, the patent discloses neither C7-/C8+ splitting nor removal of light and heavy ends prior to splitting. Finally, the patent does not suggest adjusting the cut point based on benzene or paraxylene limit points.
The '891 patent involves a split feed process, where naphtha is separated into a light fraction and a heavy fraction. This patent states that the light fraction is "subjected generally to less severe reforming conditions than a higher boiling fraction." The light and heavy fractions are defined by various "cut points," but a cut point of about 200.degree. F. or 240.degree. F. is said to be preferred, "since it is intended to concentrate substantially all C6 hydrocarbons and a substantial portion, if not a major portion, of C7 hydrocarbons into the light naphtha fraction." (Col. 2, lns 64-67.) The heavy fraction is said to be that portion boiling below about 240.degree. or 260.degree. F. (Col. 5, lns 19-21.) A preferred cut point is 250.degree. F. (Col. 14, ln 72.) The key features of the present invention, however, are not suggested. For example, in the '891 patent, a single type of catalyst is used in treating the split stream.
The '022 patent discloses splitting the naptha feed, but it also states that the C7's are combined with the heavy reformer feed. Moreover, a "typical operation," shown in a Table in columns 5 and 6, shows that the recycle stream combined with the feed to the heavy fraction reformer has 62.8% isoheptanes and 15.7% normal heptane. Furthermore, even after C5-'s are removed, the heavy fraction reformate (motor fuel 51) has an RON of only 96.5. Thus, that patent does not disclose or suggest the present invention.
As discussed below, one important aspect of the present invention is formation of a heavy fraction reformate having a surprisingly high RON, preferably 104 or above and more preferably 108 or above. As used herein, the "RON" of a material shall mean its research octane number as measured by ASTM D2699-95a. It is known that RON generally reflects the aromatics concentration, and is often used as a measurement of reformer severity. It is understood by persons skilled in the art that the RON can be boosted by manipulating operating conditions, e.g., by adjusting the temperature and/or pressure during reforming. This idea is discussed in the '826 patent. However, as acknowledged in that patent it is also known that increasing severity often leads to undesirable secondary reactions such as hydrocracking and coking, which reduce aromatics yield. In paraxylene production, such secondary reactions tend to reduce the yield of xylene precursors. Thus, typical reforming or aromatizing operations for making benzene and paraxylene products are conducted under moderately severe conditions, producing reformate with an RON of about 100, or perhaps slightly higher. Reformates as high as 103 are sometimes produced, as disclosed in the '177 patent. However, none of the references discloses boosting the RON to 104 or above or even 108 and above.
A frequent problem encountered with using petroleum naphtha as a feedstock is the unpredictable nature of the feed stream composition. The problem arises when the naphtha feed comes from more than one source depending on cost and availability. In such a situation, the petroleum naphtha feed does not have necessarily a set or predictable composition. This unpredictability is especially true of a "full boiling range naphtha," that is, naphtha hydrocarbon material boiling over one of several ranges. For example, one naphtha may boil over a range from about 104.degree. C. to about 176.degree. C. Another naphtha may boil over a range of from about 32.degree. C. to about 204.degree. C. Certain raw naphthas have a high proportion of heavy ends, such as C8's through C10's. Others have a high proportion of light ends, such as C6's and C7's. The present invention offers a method for continuously producing a high purity benzene and a high purity paraxylene while accomodating such fluctuations in naphtha feedstock composition.
Another aspect of the invention involves disproportionation of toluene, in combination with one or more of the other steps referenced above. It is known that disproportionation of toluene produces benzene and xylene. Disproportionation may be accomplished in a variety of ways. For example, the '177 patent discloses disproportionation using molecular sieve catalysts. However, in the '177 patent, and in other processes, the toluene stream is extracted before being subjected to disproportionation. Extraction removes non-aromatics that have boiling points close to toluene and are therefore difficult to remove by distillation. Toluene disproportionation is discussed in various patents, including U.S. Pat. Nos. 3,957,621; 4,052,476; 4,016,219; 4,097,543; 4,962,257; and 4,160,788. While the '621 patent refers to disproportionation of unextracted toluene, the stream containing the toluene that is treated to disproportionation is obtained from a reformate which is itself formed from a "heavy reformate" feedstock having large quantities of ethyl benzene, xylenes, and C9+ aromatics. Thus, the '621 patent fails to disclose or suggest the present invention, in which a C7- naphtha feed (light fraction) is reformed to provide a reformate rich in benzene and toluene, and where the reformate is subjected to disproportionation without further reforming, preferably after removal of benzene and xylenes.
Another aspect of this invention involves production of benzene, to surprisingly high levels of purity, e.g., up to 99.989 wt % benzene or even higher. Benzene is typically purified by aromatics extraction followed by distillation. For example, in the '177 patent, Sulfolane is used to purify a mixed aromatics stream, which is then subjected to distillation. However, there is no suggestion to clay treat or selectively hydrogenate the benzene in order to obtain a high purity product Surprisingly, with the present invention, a benzene product is obtained having a purity of about 99.989 wt % benzene or more. Surprisingly, this purity exceeds even the ASTM Refined Benzene-545 standard. Moreover, the benzene product of this invention preferably has a toluene concentration of about 40 ppm by weight or less and a non-aromatics concentration of about 70 ppm by weight or less.
Finally, another aspect of this invention is simultaneous production of high purity paraxylene from the same naphtha feedstock, e.g., a product that is up to about 99.9 wt %, or even higher, pure paraxylene. Various methods and techniques are known for producing paraxylene. Simulated moving bed liquid chromatography and fractional crystallization are often used to separate paraxylene from other xylene isomers and ethylbenzene. Other methods are disclosed in the '621 patent (col. 2, lns 23-32) and in U.S. Pat. Nos. 5,401,476, which are hereby incorporated herein by reference. Still other methods are disclosed by McPherson, in PCT Publication No. WO 96/22262, entitled "Process for Production of Paraxylene Comprising a High Temperature Crystallization With at Least One Stage and a Partial Melting of the Crystals," having corresponding U.S. application Ser. No. 08/875,278, which is hereby incorporated by reference. However, in many of these processes, the purity of the paraxylene product depends primarily on the composition of the feedstock, which usually includes a range of mixed xylenes, such as meta-xylene, ortho-xylene and para-xylene. Moreover, such processes are directed to paraxylene production, and do not involve simultaneous production of high purity benzene. Accordingly, the present invention offers an improvement to these processes.